Hitherto, there have been available electrically conductive paste products in which electrically conductive metal powders composed mainly of copper powders are incorporated in organic resins such as phenolic resins, etc. This paste, because of being capable of being heat-treated at a temperature of 300.degree. C. or lower, has been widely used for forming through-holes in printed circuit boards or as electrodes thereon.
Use of silver as an electrically conductive filler gives coats excellent in electrical conductivity and oxidation resistance, but silver is expensive and in short supply. On the other hand, copper powders, are available at low and stable prices, have an initial electrical conductivity equivalent to silver, but they copper powders are so susceptible to oxidation that a problem can arise in connection with the quality and stability of the copper powders.
Silver-coated copper-based powders have been proposed as electrically conductive filers which make up for the defect of copper powders and are virtually equivalent in terms of electrical conductivity and oxidation resistance to silver. In general, such silver-coated copper-based powders may be prepared by an electroless type of substitution plating making use of the substitution reaction between copper and silver or a reduction type of electroless plating taking advantage of the silver mirror reaction of formalin. For instance, it has been proposed to substitute the surfaces of metallic copper powders by silver using a silver complex salt solution consisting of silver nitrate, ammonium carbonate and a trisodium salt of ethylenediaminetetraacetic acid and thereby precipitate silver on them (as disclosed in Japanese Patent Publication No. Sho. 57-59283), or to coat metallic copper powders with silver using a silver complex salt solution consisting of silver nitrate, aqueous ammoniaa and a sodium salt of ethylenediaminetetraacetic acid (as set forth in Japanese Patent Kokai Publication No. Sho. 61-3802).
However, the conventional silver coating on the copper powders' surfaces by substitution and precipitation fails to achieve a complete coating of silver. Moreover, although the silver coats excel in their initial electrical conductivity, their properties, especially oxidation resistance, deteriorate with time due to impurities, like SO.sub.4 and Na ions, which result from the processes of degreasing the starting copper powders and removing oxides and unstable by-products generated during substitution and precipitation, such as Cu(OH).sub.2 and Cu.sub.2 O. Use of a silver cyanide bath poses problems in connection with safety and waste water disposal, because it is a deadly poison.
Having been accomplished with the foregoing in mind, this invention has for its object to provide a method for producing silver-coated copper-based powders inexpensively, which excel in humidity resistance and oxidation resistance, show stable properties with no change with time, and is improved in terms of electrical conductivity.